"If we think we've made enough progress, then we'll keep on the path that we're on," Tad Devine, a senior Sanders aviser, told NPR Saturday. "If we think we have to, you know, take a different way or reevaluate, you know, we'll do it then. But right now, we think the best path beyond is the one we laid out months ago."

If Sanders is to win the nomination, then he must win each coming contest by a margin of 20 points on average. Tuesday holds five contests, and polling data suggests that large victories will be hard-won at best.

In Pennsylvania, the largest contest of the night with 210 delegates, Quinnipiac University polls have Sanders trailing former Secretary of State Hillary Clinton by six points, and the same poll has him much farther behind in Connecticut and Maryland. The other two states, Rhode Island and Delaware, have no public polling information.

"There's a pathway to victory for Sen. Sanders, but I think you know it's certainly gotten brambled," Neil Sroka of the progressive group Democracy for America, which supports Sanders, also said in an interview with NPR.

As of now, Clinton's delegate lead is more than twice what President Barack Obama's was in 2008. In spite of that, Sanders' supporters such as Sroka are saying that it is important that the Vermont senator stay in the race.

"There is not a single doubt in my mind that the strong campaign that Bernie Sanders is waging right now is making the Democratic Party better, stronger and more focused on the populist progressive issues that we need to take on if we are going to be successful in November," Sroka said.

However, unless there is an unexpected sweep of contests in states like Pennsylvania and others that would sustain Sanders until June's California contest, which he would then need to win by a large margin, his campaign may need to start looking beyond the end of Democratic primaries.